JPH03265197A - Radio wave absorber - Google Patents

Abstract

PURPOSE:To have fundamental performance in such a way as to allow this absorber to be free from embrittlement even after it is used for a long period and improve wave absorbing efficiency for both a basic wave and high frequency as well by using Mn-Zn ferrite containing an excess of iron and/or magnetite and graphite in parallel. CONSTITUTION:75wt.% of Mn-Zn ferrite (average particle size: 3.5mu) containing the composition of Fe2O3: 67mol%, ZnO: 18mol%, MnO: 15mol%, i.e., a kind of ferrite containing an excess of iron, is used as soft ferrite. Further, 0, 2.0, 3.5, 5.0wt.% of graphite (average particle size: 7mu, true density: about 2.2g/cm<2>) are used in combination and poly methyl pentene, poly butene-l resin and Topanol CA (antioxidant) are used for the rest of this compound after mixing them uniformly in a state of Tg=13 deg.C by an extruding kneader. After cooling for solidification, the above compound is pulverized to make pellets. Then, injection molding is carried out to form this product into complicated shapes; besides, its embrittlement is prevented even though it is used for a long period in a hot and humid environment and superior radio wave absorption over a wide frequency band is obtained as well.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is a radio wave absorber used in high temperature and/or humid environments, particularly for use in preventing radio wave leakage in microwave ovens equipped with external heating means. Regarding radio wave absorbers.

[Prior art] A radio wave absorber for absorbing leakage radio waves from a microwave oven is
There are two shapes suitable for use in a microwave oven window frame, but since the shapes are quite complex, they are usually injection molded. Therefore, thermoplastic resins suitable for injection molding, such as polyethylene, polypropylene, and polyamide, have been used as binders. These resins, manganese zinc ferrite (hereinafter referred to as Mn-Zn ferrite), nickel zinc ferrite (hereinafter referred to as Ni-
Radio wave absorbers for conventional microwave ovens are made by homogeneously mixing soft ferrite such as Zn ferrite (referred to as Zn ferrite) or magnetic powder such as magnetite.
) has been designed to absorb well. However, radio waves leaking from a microwave oven include harmonics in addition to the fundamental wave having the maximum output, and it is known that these harmonics cause radio wave interference like the fundamental wave. In particular, since the fifth harmonic (12.25 GHz) overlaps with the frequency band of the carrier wave of satellite broadcasting (11.5 to 12.5 GHz), a radio wave absorber that particularly strongly absorbs this component is required.

In order to meet these demands, reducing MnO and F
Mn-Zn ferrite with increased ezes (hereinafter simply referred to as iron-rich ferrite) has been developed (Japanese Unexamined Patent Publication No. 1983-1999).
-250603). The radio wave absorption performance of this ferrite tends to increase dramatically as the harmonics increase in the range of 2 to 12 GH2, reaching a maximum at 10 GHz and reaching 12 GHz.
z, the radio wave absorption performance is slightly lower, but it still has a sufficiently high radio wave absorption performance, and therefore, if this is used, a radio wave absorber with excellent performance in the harmonic band can be obtained, as disclosed in JP-A-61-73400. Disclosed in the official gazette. This publication also states that this ferrite has slightly inferior absorption characteristics to the fundamental wave compared to conventional ferrite.

Although this does not necessarily significantly impair its practicality as a radio wave absorber, considering the fact that the fundamental wave is the component with the maximum output, it is still preferable to absorb more of the fundamental wave component. Needless to say.

[Problem to be solved by the invention]

By the way, as mentioned above, no matter how much you improve the radio wave absorption performance in the fundamental wave and harmonic bands by devising the component composition of the magnetic powder, as long as you use a known binder such as polyethylene, polypropylene, polyamide, etc. ,
It lacked a balance in heat resistance, flame retardance, moisture resistance, mechanical strength, etc., and could not be obtained that was practically satisfactory. For example, when placed in a high temperature state, it deforms, and when placed in a high temperature and humid environment, such as in a microwave during use, there is a risk of moisture absorption and deformation. This has been a serious problem especially when used as a leakage radio wave preventive device for recent microwave ovens equipped with external heating means other than microwave heating such as infrared heating means and steam heating.

In order to solve this problem, a radio wave absorber using polymethylpentene as a binder resin has been proposed (Japanese Utility Model Application No. 56195/1983). However, when used for a long period of time, polymethylpentene crystallized and became brittle, making it unable to withstand the stress caused by dimensional shrinkage and causing it to crack.

[Means to solve the problem]

As one solution to this problem, the present inventors discovered a method of using polymethylpentene with a glass transition point (Tg) of 20°C or less, preferably together with a softener, and disclosed this technology in the "Radio Wave Absorber" ” (filed on February 26, 2013).

The purpose of the present invention, as in the above application, is to provide a radio wave absorber that has basic performance that does not become brittle even after long-term use, and in addition, has extremely high radio wave absorption performance for both fundamental waves and harmonics. We aim to provide the following.

As a result of extensive research, the present inventors have found that if iron-rich Mn-Zn ferrite and/or magnetite and graphite are used together as radio wave absorbing materials, extremely high radio wave absorption performance for both fundamental waves and harmonics can be achieved. The present invention was achieved by discovering that the following can be obtained.

[Function] The reason why the iron-rich FIn-Zn ferrite and magnetite (Fe+On) used as the magnetic powder in the present invention particularly well absorbs radio waves of 10 GHz or more is not known in detail, but
It is presumed that this is because both have higher saturation magnetic flux density and Curie temperature than normal Mn-Zn ferrite, resulting in increased magnetic permeability loss. Particularly in the case of magnetite, it is thought that one of the reasons for its excellent radio wave absorption properties is that each of them inherently has high electrical conductivity. Preferred F of iron-rich Mn-Zn ferrite
For convenience, the compositions of e, Mn, and Zn are shown in the form of oxides: FezO: +: 65-85 molχ, MnO: 2-
20IllO1χ, ZnO: 10 to 20 mo1%. Outside this range, harmonic absorption performance deteriorates. It is desirable that both the ferrite powder and the magnetite powder have an average particle diameter of 1 to 10 degrees Cron as determined by the Fischer subsieve sizer method, and have a wide coarse distribution. Otherwise, kneading and injection molding will be difficult or the surface smoothness of the molded rod will be impaired. Furthermore, especially in the case of magnetite powder, it is desirable that each of the particles be a single crystal rather than an aggregate of fine particles. We have confirmed through experiments that if this is not the case, the radio wave absorption performance will be inferior even if the composition is the same. The reason for this is probably that electrical conductivity is impaired.

Graphite is a hexagonal crystal of carbon (also known as graphite), and it is widely known that so-called "carbon" is an excellent radio wave absorbing material due to its conductivity, and in fact, it is an excellent radio wave absorbing material. (Japanese Patent Publication No. 62-6680, Japanese Patent Publication No. 64038-1983, Japanese Patent Publication No. 249392-1982, etc.). However, what has traditionally been called "carbon" has a very small particle size (
several hundred angstroms), so-called "carbon black." Although this material is crystallographically graphite, it is incompletely crystallized and has an extremely large specific surface area, so it cannot be made into a radio wave absorbing molded product unless it is kneaded with an extremely viscous organic polymer such as chloroprene rubber. I don't get it. In other words, injection molding is virtually impossible, and injection molding resins such as polymethylpentene cannot be used as a binder, and of course, the excellent properties of polymethylpentene cannot be utilized.

In fact, according to experiments conducted by the present inventors, kneading carbon black with a BET specific surface area of approximately 1000 rrf /g into polymethylpentene together with ferrite powder results in the following effects:
Even if the amount of carbon black is as small as a few percent by weight, it is extremely difficult to do so, and the fluidity of the molten mixture is also extremely low, making injection molding almost impossible. Therefore, the inventor of the present invention abandoned the use of carbon black and instead used graphite, whose crystal structure is sufficiently developed to give a clear X-ray diffraction pattern, and obtained unexpectedly good results.

In other words, if graphite is combined with iron-rich Mn-Zn ferrite and/or magnetite, the radio wave absorption performance will be dramatically improved just by adding a small amount of graphite, and the mechanical strength of the formed rod will be reduced as in the conventional method. We have discovered that sufficient radio wave absorption performance can be achieved without incorporating a large amount of ferrite. It has also been found that since the fundamental wave absorption characteristics are improved, the inherent drawback of this ferrite, which is low radio wave absorption performance for fundamental waves, can be compensated for. However, since graphite particles have an extremely flat shape,
The fact remains that they are not entirely suitable for injection molding and tend to inhibit the flowability of the melt. In addition, when the amount of graphite added is increased, the mechanical strength of the formed rod decreases, although this is not the case when carbon black is added. According to the research conducted by the present inventors, in order to balance the performance and injection moldability of the radio wave absorber of the present invention, it is necessary to use graphite with an average particle size of 1 to 30 microns as measured by a laser, and to add the The sum of the weight percent of ferrite and the weight percent of graphite is 60.
It has been found that it is desirable to set the content to 85% by weight. If it exceeds 85% by weight, the strength of the molded bar will be too low, and if it is less than 60% by weight, the radio wave absorption performance will be too low, making it unsuitable for practical use.

The polymethylpentene used in the present invention has a Tg lowered to 20"C or less by lowering the regularity of the molecular arrangement, that is, lowering the crystallinity. By using this, the dimensional shrinkage rate can be reduced. It is possible to obtain a radio wave absorber that is small in size and does not become brittle even when exposed to high temperatures repeatedly over a long period of time, and therefore does not crack.Of course, the inherent advantages of polymethylhentene are low water absorption and polyamide. Needless to say, it also maintains higher heat resistance than polypropylene.

Addition of a softener imparts appropriate flexibility to the molded rod, which helps prevent cracking and chipping during product handling and improves injection moldability. Polylaten-1 resin is one of the effective softeners.

Incidentally, as in the case of ordinary resin molding, in addition to the above-mentioned components, a binder stabilizer is added to the actual radio wave absorber in an amount of 1 to 5% by weight of the binder. These include, for example, monophenolic antioxidant stabilizers (trademark Nilganox, Ciba Geigy), polymeric phenolic antioxidants (such as dilauryl thiodipropionate,
Trade name: DLTP, Yoshitomi Pharmaceutical Co., Ltd.), etc.

〔Example〕

Examples will be shown next, but these are only examples of embodiments of the present invention and do not limit the scope of the present invention in any way.

(Example 1) As soft ferrite, FezO3: 67 mol%, ZnO: 18 molχ, which is a type of iron-rich ferrite,
MnO: 15m.

75% by weight of Mn-Zn ferrite (average grain size 3.5 microns) having a composition of 1z, the average grain size measured by laser is 7 microns, and the true density is about 2.2
g/c11Y graphite (manufactured by Tokai Carbon Co., Ltd.)
12,0,3,5,5,0% by weight, the remainder being polymethylpentene and polybutene-1 resin manufactured by Mitsui Petrochemical Co., Ltd. with Tg = 13°C (composition ratio of these three is 80:20 by weight)
), and the antioxidant dopanol CA (binder 3
% by weight) were uniformly mixed using an extrusion kneader, cooled and solidified, and then ground to make pellets. This was injection molded at 240°C to create plate-shaped test pieces, and their 2.45 GHz and 12.2 GHz
The radio wave attenuation constant at 5 GHz was measured using a network analyzer. In addition, long-term annealing (
The elongation at break and dimensional shrinkage after 500 hours at 160°C were also measured. These results are shown in Table 1. Furthermore, when the heat resistance of the binder of the present invention was evaluated in terms of melting point, it was found that the heat resistance was 220°C, which was much higher than 180°C for polyamide and 130°C for polypropylene.

(Example 2) Magnetite (average grain size 1.5
75% by weight of this magnetite.
A test piece was prepared in the same manner as in Example 1 except for the following. Table 2 shows the radio wave attenuation constant.

(Example 3) Iron-excess 719 used in Example as soft ferrite
4137.5% by weight and magnetite 3 used in Example 2
A test piece was prepared with the same composition as in Example 1 except that a mixture with 7.5% by weight was used. Table 3 shows the radio wave attenuation constant.

(Example 4) The iron-rich ferrite used in Example 1 and the magnetite used in Example 2 were used as soft ferrite, and their contents were reduced to 65% by weight, and graphite was added to the iron-rich ferrite and magnetite, respectively. 5% by weight of iron-rich ferrite and 32.5% by weight of iron-rich ferrite.
A total of three types of test pieces were prepared by adding 5% by weight of graphite to a mixture of 32.5% by weight of magnetite and 32.5% by weight of magnetite, using the same types and amounts of the binder and antioxidant as in Example 1. Table 4 shows their radio wave attenuation constants.

(Comparative example) F, which is a type of conventional Mn-Zn ferrite
c40z: 54 mol%, ZnO: 14 molχ, Mn
A radio wave absorber was prepared in the same manner as the product of the present invention in Example 1, except that 75% by weight of Mn-Zn ferrite (average particle size: 3.3 microns) having a Mi content of O:32mol1% was used. Table 5 shows the radio wave attenuation constant.

As is clear from Tables 1, 2, and 3, by using iron-rich MnZn ferrite and/or magnetite, it is possible to dramatically improve the radio wave attenuation constant just by adding a small amount of graphite. Furthermore, the elongation at break is much larger than the dimensional shrinkage rate, which means that it will not break even if stress associated with dimensional shrinkage is applied. Furthermore, as can be seen from the comparison between Tables 4 and 5, iron-rich ferrite and/or
Alternatively, if magnetite and graphite are combined, the same level of radio wave absorption performance can be obtained with a smaller amount of inorganic filler (in this case, soft ferrite and graphite) than when using normal ferrite, so it has high strength, which means that it will not crack or chip when handled. You can obtain a product that is less likely to cause

Table 1 Table of products using iron-excess ferrite 2 Table of products using magnetite 5 Table of products using conventional ferrite 3 Table of products using a mixture of iron-excess ferrite and magnetite 4 *) Iron-excess ferrite 32.5% by weight, magnetite 3
2.5% by weight [Effects of the Invention] As described above, according to the present invention, injection molding is possible and it is possible to mold into a complicated shape, and even when used for a long time in a hot and humid environment. Does not become brittle,
Moreover, it is possible to obtain a radio wave absorber having extremely excellent radio wave absorption performance over a wide range of frequency bands from fundamental waves to harmonics. Therefore, for microwave ovens that have an external heating function other than microwave heating, such as the microwave oven function that has become popular in recent years, there is a strong need for countermeasures against leakage radio waves, especially against the fifth harmonic as well as the fundamental wave. Particularly suitable. Furthermore, for applications where the radio wave absorption performance does not need to be so high, it is possible to sufficiently cope with the use by reducing the amount of graphite blended. This radio wave absorber uses polymethylpentene, which has a glass transition point of 20°C or lower, as a binder, so when the amount of graphite is reduced, the radio wave absorber can be replaced with the conventional radio wave absorber using polymethylpentene. It can be made into a product with higher strength compared to Note that the radio wave absorber of the present invention can of course be applied to prevent radio waves from devices other than microwave ovens, as long as the temperature and/or humidity environment is the same as that of microwave ovens.

Claims (5)

[Claims] (1) Mn-Zn ferrite powder with more Fe content and less Mn content than normal Mn-Zn ferrite and/or
Or, a homogeneous mixture of soft ferrite powder consisting of magnetite powder and graphite powder with a glass transition point of 20
A radio wave absorber whose main component is polymethylpentene uniformly dispersed at temperatures below ℃. (2) The radio wave absorber according to claim 1, in which a softening agent of polymethylpentene is added. (3) The radio wave absorber according to claim 2, wherein the softener for polymethylpentene is polybutene-1 resin. (4) As soft ferrite, the average particle diameter is 1 to 10
micron, and the proportions of Fe, Mn, and Zn expressed as oxides are Fe_2O_3: 65-85 mol%, MnO: 2
~20 mol%, ZnO: 10-20 mol%
The radio wave absorber according to claim 1, characterized in that n-Zn ferrite or a mixture of the Mn-Zn ferrite and magnetite is used. (5) The radio wave absorber according to claim 1, wherein the graphite powder has an average particle diameter of 1 to 30 microns.